## The Crystal Structure of Julgoldite

Rudolf Allmann and Gabrielle Donnay

Fachbereich Geowissenschaften, Philipps-Universität, 355 Marburg, Germany

Department of Geological Sciences, McGill University, Montreal 101, Quebec, Canada

**Summary:** The structure of julgoldite, Ca(Fe1−x2+Fex3+)Fe23+Si3O10+x(OH)4−x(x≈0·5), has been determined and refined to an R value of 4·6 % based on 2417 symmetry-independent reflections, of which 765 are unobserved (weighted *R* = 4·3%). The cell dimensions are *a* 8·922(4), *b* 6·081(3), *c* 19·432(9) Å, β 97·60(6)°, the space group is *A2/m* with *Z* = 4 and D_{cale} 3·56 g cm^{−3}. Julgoldite is isostructural with pumpellyite; the Si-O and Ca-O distances of julgoldite are identical, within experimental accuracy, with corresponding distances in pumpellyite; the Fe-O distances of julgoldite are about 0·09 Å longer than the corresponding (A1, Mg)-O distances in pumpellyite. The four hydroxyl groups and the extent of O-for-OH substitution are identified by valence-sum calculations.